Preheat oven to 375°F (190°C). Grease a medium baking dish (about 8×8 inches).
Sauté the onion and garlic: In a saucepan, melt butter over medium heat. Add onion and cook until soft, about 3 minutes. Stir in garlic and cook 30 seconds more.
Make the sauce: Stir in flour and cook for 1 minute. Gradually whisk in milk and cream, stirring constantly until thickened (3–5 minutes).
Add seasonings and cheese: Stir in salt, pepper, thyme, and half of the cheddar and Parmesan cheeses. Mix until smooth and creamy.
Layer the zucchini: Spread a thin layer of sauce on the bottom of your baking dish. Add half of the zucchini slices, then half of the remaining sauce. Repeat layers, finishing with sauce on top.
Top and bake: Sprinkle the remaining cheeses (and breadcrumbs, if using) on top. Bake uncovered for 25–30 minutes, until golden and bubbly.
Cool slightly before serving so the sauce can set.
💡 Tips
Pat the zucchini slices dry with a paper towel to prevent excess water in the dish.
Add a sprinkle of red pepper flakes for a mild kick.
Try swapping cheddar for Gruyère or Monterey Jack for a twist!
Would you like me to give you a low-carb version with no flour and less cream (still super creamy)?
